(A) The topic of the present study is the structural modification of DNA double helix caused by binding of carcinogens and mutagens. It was found that there were three different types of in vitro binding of benzo(a)pyrene (BaP) metabolite, 7,8 diol 9,10 epoxide BaP, to superhelical circular DNA of plasmid Col El and SV40 virus: (1) The covalent binding which does not cause severe steric hindrance on the DNA double helix. The binding with guanine belongs in this category. (2) The binding which causes local denaturation resulting in conformational change of DNA. The binding with adenine is mostly responsible for this change. (3) The binding which breaks phosphodiester bonds. This is caused by the binding with either DNA basis and/or phosphate. The latter is stimulated by the presence of certain metal ions such as lantanium. (B) The binding characteristics were studied by using double-stranded synthetic polymers. The results supported the previously described binding forms. (C) The binding of BaP metabolites in culture cells is under investigation. BIBLIOGRAPHIC REFERENCE: Kakefuda, T., Lovinger, G.G., Gilden, R.V., and Hatanaka, M.: Electron microscopic studies of circular DNA in mouse embryo fibroblast infected by Rauscher leukemia virus. J. Virol. 21: 792-795, 1977.